XML-based captions are based on the Extensible Markup Language, and can come in the following formats:
Each of these formats is described below. For a more detailed overview of XML-based captions, including the history of how the various standards were formed, read our History of XML Captions page.
TTML is a standard for XML captions developed by the World Wide Web Consortium (W3C) in order to unify the increasingly divergent set of existing caption formats. It was designed to incorporate of all functionality of existing formats, and therefore, become the standard interchange format between applications.
TTML is a base standard in the sense that it defines a set of requirements that other more specialized XML standards can derive from. Derived standards, or Profiles as the W3C refers to them, do not need to implement all of the TTML requirements. TTML files can have the file extension (.ttml) or (.xml).
Learn more about the TTML format.
DFXP is a set of three specializations, or “Profiles” of TTML requirements: DFXP Presentation, DFXP transform, and DFXP full. The Presentation profile is for video players, and the Transform profile is used for video editing. The Full profile includes all the features defined in the base TTML standard. Hence, the terms TTML and DFXP are often used interchangeably. In practice, however, TTML files may have slight variations in format, whereas the DFXP files are generally consistent. DFXP files end with the file extension .dfxp.
DFXP is the preferred caption format for Adobe Flash, Microsoft Silverlight, YouTube, and many other online video providers.
Learn more about the DFXP format.
SMPTE-TT is an extension of TTML created by the Society of Motion Pictures and Television Engineers (SMPTE).
SMPTE concluded that the TTML standard addressed many of the needs of the TV and film industries, but the feature set is not sufficient to address all of those needs. In particular, it lacked support for bitmap images (needed for certain European caption formats), binary payloads (needed to support the existing CEA-708 standard for live broadcast), and the ability to adapt the presentation style based on the playback device.
To accommodate these additional requirements, the SMPTE created a new TTML profile which extended the TTML standard to meet their requirements. The new profile, called SMPTE-TT, encompasses the DFXP Full profile as well as the new extensions. SMPTE-TT files end with the file extension .xml.
SMTPE-TT is popular in the broadcast television industry, because the FCC designated it as a “safe harbor interchange and delivery format” for online captioning.
Learn more about the SMTPE-TT format.
If your project requires a specialized version of XML-based captions, please contact us, and we’d be happy to discuss it with you.